1. Field of the Invention
The present invention relates to a transmission. More particularly, to a synchronizer-mechanism-equipped transmission.
2. Description of Background Art
A synchronizer mechanism for a transmission is known. See, for example, Japanese Patent Laid-Open No. 2004-125112.
In the synchronizer mechanism disclosed in Japanese Patent Laid-Open No. 2004-125112, ring gear teeth (ring teeth) formed on an outer ring (synchronizer ring) of a blocking ring come between gear teeth (gear dog teeth) and spline teeth (sleeve teeth). The gear teeth are formed on a first transmission gear that is pivotally supported in a relatively rotatable manner on each of main and counter shafts, each being a rotating shaft. The sleeve teeth are formed on a sleeve (synchronizer sleeve) that is pivotally supported on the rotating shaft in such a manner so as to be restricted in relative rotation and axially movable. During gear shifting, the synchronizer sleeve moves, causing the sleeve teeth to come into contact and mesh with the ring teeth first and then come into contact and mesh with the gear dog teeth. This causes the synchronizer sleeve (and the rotating shaft) and the first transmission gear to be synchronized and coupled together.
A mechanism that includes a shift drum and a shift fork is generally known as a transmission drive mechanism for moving the synchronizer sleeve with an actuator so as to shift gears by achieving synchronization and coupling using a synchronizer mechanism. The shift drum is driven to rotate by an actuator. The shift fork is guided by a lead groove of the shift drum to move axially as a result of rotation of the shift drum. The shift fork engages with the synchronizer sleeve, thus moving the synchronizer sleeve. See, for example, Japanese Patent Laid-Open No. 2008-215555.
Then, the transmission drive mechanism has a detent mechanism to position the shift drum at a predetermined rotation position for each gear position.
A detent mechanism normally has a star cam. An uneven cam surface is formed on an outer circumferential end surface of the star cam. Detent recessed portions, associated with gear positions, and tapered and pointed projecting portions are circumferentially and sequentially formed on the uneven cam surface in such a manner as to alternate continuously. The star cam is provided to rotate integrally with the shift drum. Biasing means causes a roller to come into contact with and press the uneven cam surface of the star cam, causing the roller to slip into the required detent recessed portion. As a result, the shift drum is rotated and biased together with the star cam, thus positioning the shift drum at a predetermined rotation position. See, for example, Japanese Patent Laid-Open No. 2008-215555.
When the shift drum is driven to rotate by the actuator during gear shifting, the star cam of the detent mechanism rotates integrally with the shift drum. At the same time, the synchronizer sleeve is moved by the shift fork that has been guided into the lead groove of the rotating shift drum, causing the sleeve teeth of the synchronizer mechanism to come into contact and mesh with the ring teeth and then come into contact and mesh with the gear dog teeth. As a result, the synchronizer sleeve and the transmission gear are synchronized and coupled together.
The frictional resistance to which the synchronizer sleeve is subjected varies from one stage to another during the time period from when the sleeve teeth come into contact and mesh with the ring teeth to when the sleeve teeth come into contact and mesh with the gear dog teeth in the course of the movement of the synchronizer sleeve driven by the actuator.
On the other hand, while the synchronizer sleeve moves, the star cam that is provided integrally with the shift drum rotates with the roller pressed onto the uneven cam surface by the biasing means. As a result, the roller that is pressing a pre-gear-shifting detent recessed portion climbs over the projecting portion of the star cam thanks to the rotation of the star cam, causing the roller to press a post-gear-shifting detent recessed portion.
Therefore, the biasing force exerted by the biasing means to press the roller onto the uneven cam surface of the star cam is opposite in direction between before and after the roller climbs over the projecting portion of the star cam.
If actuator driving stops for some reason while the star cam is driven to rotate and move the synchronizer sleeve, and if, as a result, the force exerted by the actuator to rotate the shift drum, i.e. the star cam, is lost, the frictional force to which the synchronizer sleeve is subjected is larger than the biasing force exerted by the biasing means to move the synchronizer sleeve by rotating and biasing the star cam and the shift drum, depending on the position of the synchronizer sleeve of the synchronizer mechanism. As a result, the shift drum may stop at a rotation position halfway through shifting.
When the shift drum stops at a rotation position halfway through shifting because actuator driving stops, and if, at this moment, the synchronizer sleeve of the synchronizer mechanism is located where the sleeve teeth come into contact with the ring teeth, the tips of the teeth continuously butt each other, which is not preferred.